Selective cap layers over recessed polysilicon plugs

ABSTRACT

Methods are provided for selective formation of oxidation-resistant caps for conductive plugs in semiconductor device fabrication. One embodiment of the present invention forms a sacrificial layer over a recessed polysilicon plug. The sacrificial layer is readily planarized using chemical mechanical planarization to isolate the cap within a recessed via. Then, an immersion plating process is used to replace the atoms of the sacrificial layer with atoms of a desired metal, such as platinum, thereby creating a metal cap isolated within the via. The advantages of planarization to isolate material within recessed via are thus obtained without having to planarize or otherwise etch the desired metal. The cap layer can be further reacted to form a barrier compound prior to forming a capacitor over the plug. Advantageously, the plug structure resists oxidation during fabrication of overlying capacitors that incorporate high dielectric constant materials.

FIELD OF THE INVENTION

This invention relates generally to integrated conductive plugscontacting semiconductor elements and, more particularly, tooxidation-resistant partial plugs (e.g., oxygen barriers and conductiveoxides).

BACKGROUND OF THE INVENTION

Semiconductor devices formed in the surface region of a silicon wafersubstrate each have multiple elements to be electrically connected tothe surrounding circuitry and to each other. Some of these electricalconnections extend through protective insulating layers that cover eachdevice level to electrically isolate adjacent levels. The insulatinglayers typically provide planarized surfaces for subsequentsemiconductor device fabrication. Insulating materials includeborophosphosilicate glass (BPSG), oxide deposited fromtetraethyl-orthosilicate (TEOS), newer low dielectric (low k) materials,etc.

For example, a memory cell in an integrated circuit, such as a dynamicrandom access memory (DRAM) array, typically comprises a charge storagecapacitor (or cell capacitor) electrically connected to an access devicesuch as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET).The MOSFET applies or removes charge on the capacitor, thus affectingthe logical state defined by the memory cell. After formation of theMOSFET device elements, a protective insulating layer of BPSG istypically deposited, through which electrical connection must then bemade to the subsequently fabricated capacitors and wiring layers abovethe BPSG layer. It is important to maintain good ohmic electricalconnections between the capacitors and the underlying device elements(e.g., drain region of a MOSFET), and to maintain these good ohmiccontacts throughout the lifetimes of the integrated circuits. Moreover,the material that contacts the substrate must be compatible so as not topoison the active areas and disturb finely tailored electricalcharacteristics.

Typically, vertical connections are made by filling vias formed throughinsulating layers with conductive polycrystalline silicon (i.e.,polysilicon or poly), thereby contacting the substrate. The resultantstructure filling the via is often referred to as a “poly plug.”

In order to maintain a certain minimum charge storage as devicedimensions are scaled down and packing densities increase, capacitorsfor DRAM devices are being developed for incorporation of dielectricmaterials having increased dielectric constants (k). Such high kmaterials include tantalum oxide (Ta₂O₅), barium strontium titanate(BST), strontium titanate (ST), barium titanate (BT), lead zirconiumtitanate (PZT) and strontium bismuth tantalate (SBT). These materialsare characterized by effective dielectric constants significantly higherthan conventional dielectrics (e.g., silicon oxides and nitrides).Whereas k equals about 3.9 for silicon dioxide, the dielectric constantsof these new materials can range from 20 to 40 (tantalum oxide) to 300(BST), and some even higher (600 to 800). Using such materials enablesthe creation of much smaller and simpler capacitor structures for agiven stored charge requirement, enabling the packing density dictatedby future circuit design.

Difficulties have been encountered, however, in incorporating thesematerials into fabrication process flows. For example, Ta₂O₅ isdeposited by chemical vapor deposition (CVD) employing organometallicprecursors in a highly oxidizing ambient. Additionally, after depositionthe material is annealed to remove carbon. This anneal is typicallyconducted in the presence of nitrous oxide (N₂O), which is also highlyoxidizing, while volatile carbon complexes are driven out.

Due to the oxidizing nature of the reactants and by-products for forminghigh-k materials, surrounding materials are subject to degradation.Similarly, formation of other high k materials often involves exposingadjacent materials to oxidizing or otherwise corrosive environments.Corrosion of the conductive materials forming the electrical connectionsto device elements reduces their conductivity, and has been viewed as amajor obstacle to incorporating high-k materials into integratedcircuits.

Accordingly, a need exists for improved contact plugs and methods ofmaking the same.

SUMMARY OF THE INVENTION

The present invention is a method of selectively forming materials inconductive plugs in a manner that avoids etching the material. In theillustrated embodiments, a noble metal, preferably platinum, isincorporated into a cap to a conductive plug. In one embodiment, asacrificial layer is deposited over a recessed silicon plug. Thesacrificial layer is readily etched, particularly by chemical mechanicalplanarization, to isolate the material within a via. The layer is thenreplaced, preferably by immersion plating, with the noble metal.Alternatively, the refractory metal layer can be directly formed byselective deposition, particularly electroless plating, which does notrequire the formation of a sacrificial layer.

Thus, in accordance with one aspect of the invention, a method isprovided for selectively forming a cap layer for a conductive plug. Avia, exposing a semiconductor substrate, is formed in a semiconductorsubstrate. A partial plug then partially fills the via to directlycontact the substrate. A sacrificial layer is deposited into the via, inelectrical contact with the partial plug, and over the insulating layer.Excess sacrificial metal layer is removed from over the insulatinglayer. Atoms of the sacrificial metal layer within the via are replacedwith atoms of a cap material.

In accordance with another aspect of the invention, a method is providedfor fabricating an integrated circuit. The method includes filling acontact via in an integrated circuit with a silicon plug. The plug isrecessed, and a plug cap including a noble metal is selectively formedover the recessed silicon plug without etching the noble metal. Acapacitor is then formed over the plug cap, the incorporating high kdielectric material.

In accordance with another aspect of the invention, a process isprovided for forming an integrated circuit memory cell. A recessed plugis formed within a via through an insulating layer. The recessed plug isthereby electrically connected to a transistor active area. A cap layer,comprising non-oxidizing material, such as an oxygen barrier or aconductive oxide, is selectively formed within the via over the recessedplug. A capacitor with a high dielectric constant material is thenformed over the cap layer.

In accordance with another aspect of the invention, a method is providedfor forming a plurality of non-oxidizing contact structures in anintegrated circuit. The method includes forming a plurality of openingsthrough an insulating layer, and then blanket depositing a sacrificialmaterial over the insulating layer and into the openings. Thesacrificial layer is etched to remove it from over the insulating layeroutside the openings, and the sacrificial layer left inside the openingsis replaced with a non-oxidizing material, such as an oxygen barrier ora conductive oxide.

In accordance with another aspect of the invention, a memory cell isprovided in an integrated circuit. The cell includes a silicon partialplug that partially fills a via formed through an insulating layer. Thepartial plug contacts an underlying semiconductor substrate. A plug capcomprising a noble metal fills a top portion of the via over the partialplug. A capacitor with a high dielectric constant material is positionedover the plug cap.

In accordance with another aspect of the invention, an integratedcircuit is provided with a conductive plug. The plug includes a partialplug that is compatible and in electrical contact with an underlyingsemiconductor substrate. A platinum-containing cap layer is aligned overthe partial plug, making electrical contact with the partial plug. Aninsulating layer surrounds the partial plug and cap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent to the skilledartisan from the following detailed description read in conjunction withthe appended drawings, which are meant to illustrate, and not to limit,the invention, and in which:

FIG. 1 schematically illustrates a typical MOSFET device structure in apartially fabricated integrated circuit, in accordance with a startingpoint for the preferred embodiment of the present invention;

FIG. 2 schematically illustrates the integrated circuit of FIG. 1following formation of an insulating layer, a via etched therethrough,and a polysilicon plug partially filling the via;

FIG. 3 schematically illustrates the integrated circuit of FIG. 2,following deposition of a sacrificial layer onto the first insulatinglayer and into the via;

FIG. 4 schematically illustrates the integrated circuit of FIG. 3, wherethe sacrificial metal layer has been confined to the via;

FIG. 5 schematically illustrates the integrated circuit of FIG. 4, wherethe sacrificial metal layer has been replaced by another metal layer,forming a plug cap;

FIG. 6 schematically illustrates the integrated circuit of FIG. 5, wherethe plug cap has been alloyed to produce a barrier material;

FIG. 7 schematically illustrates the integrated circuit of FIG. 6, wherea second insulating layer has been formed over the first insulatinglayer and the plug cap;

FIG. 8 schematically illustrates the integrated circuit of FIG. 7, wherea substantially vertical container-shaped via has been formed in thesecond insulating layer, exposing the plug cap;

FIG. 9 schematically illustrates the integrated circuit of FIG. 8, wherea bottom electrode layer has been deposited and isolated to line thevia;

FIG. 10 schematically illustrates the integrated circuit of FIG. 9,where a high-k dielectric layer has been deposited over the bottomelectrode;

FIG. 11 schematically illustrates the integrated circuit of FIG. 10,where a top electrode layer has been deposited over the high-kdielectric layer; and

FIG. 12 schematically illustrates the structure of FIG. 11 followingdeposition of a third insulating layer, isolating the memory cellsacross the array from metal wires to be formed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments describe methods of forming conductiveinterconnections with selectively formed non-oxidizing, oxygen barrieror conductive oxide plug caps, which incorporate noble metal layers,beneath high k capacitors of an advanced DRAM circuit design. Personsskilled in the art will appreciate, in view of the disclosures herein,however, that the methods and structures disclosed herein will haveapplication in a variety of contexts, particularly where conductiveinterconnections in integrated circuits are to make contact betweenelements of different chemical and electrical characteristics.

In one preferred embodiment of the present invention, the plug caps areformed by depositing a sacrificial layer over a recessed plug,planarizing to leave the sacrificial layer only in the via, andreplacing the sacrificial layer with a second conductive material usingan immersion plating process. In another embodiment of the presentinvention, the plug caps are formed by a selective electroless platingprocess.

FIG. 1 schematically illustrates a partially fabricated integratedcircuit, comprising a MOSFET overlying a semiconductor substrate 12.While the illustrated semiconductor substrate 12 comprises anintrinsically doped monocrystalline silicon wafer, it will be understoodby one of skill in the art of semiconductor fabrication that in otherarrangements, the semiconductor substrate 12 can comprise other forms ofsemiconductor layers which include active or operable potions ofsemiconductor devices. Several device elements not relevant to thediscussion here, such as field oxide elements to isolate the activeareas of different transistors, are omitted for simplicity.

The transistor comprises source 14 and drain 16, active areas formedwithin the substrate 12 and a gate structure 18 overlying the substratebetween the active areas 14, 16. The width of the gate structure 18 ispreferably less than about 0.25 μm for current and future generationintegrated circuits. Scaling down of the gate width leads to smallerfootprints available for capacitor plates, deeper and narrower contacts,smaller metal line widths and spacing, etc.

A first insulating layer 20, typically comprising a form of oxide suchas borophosphosilicate glass (BPSG), is deposited over a plurality oftransistors 10 (one shown) across the substrate 12. In order to operatethe MOSFET device structure 10, electrical connection is made throughthe first insulating layer 20 from the MOSFET to other electricalelements in the integrated circuit. For example, to fabricate a DRAMcell, the MOSFET's drain region 16 is electrically connected to acapacitor. Depending upon the presence or absence of other circuitelements, the first insulating layer 20 can have a thickness betweenabout 0.5 μm to 2.0 μm. For example, certain DRAM circuit designs callfor “buried” digit lines running below the cell capacitors, such that athicker first insulating layer 20 is required to electrically isolatethe digit line from the underlying transistors and the overlyingcapacitors.

FIG. 2 schematically illustrates an interim step in the fabricationprocess of a DRAM cell, in accordance with the preferred embodiment. Asubstantially vertical via 30 is formed through the first insulatinglayer 20 to expose the drain region 16 below. One possible method offorming the via 30 is to perform a contact etch through the firstinsulating layer 20, ending at the drain region 16. The illustrated via30 is substantially circular in cross-section, with a width or diameterpreferably between about 0.10 μm and 0.5 μm, more preferably betweenabout 0.15 μm and 0.25 μm. The via can, however, have other geometriesand dimensions without departing from the spirit and scope of thepresent invention.

In the illustrated embodiment, where the via opens to the substrate, thevia 30 is preferably partially filled by a partial plug 32, comprisingan initial conductive material, to electrically contact the drain region16. In the illustrated embodiment, the partial plug 32 comprisesconductively doped polycrystalline silicon (“polysilicon” or “poly”),and can be formed by any suitable processes. Advantageously processesare well developed for depositing polysilicon into deep, narrow contactvias with good step coverage using chemical vapor deposition (CVD).Moreover, polysilicon has a relatively high melting point, enabling itto withstand further front-end high temperature processing. In otherarrangements, the partial plug 32 can comprise tungsten or metalsilicide. Like polysilicon, CVD processes for depositing tungsten,tungsten silicide (WSi_(x)) and titanium silicide (TiSi_(x)) are welldeveloped.

In the illustrated embodiment, the partial plug 32 is formed first by ablanket deposition, followed by recessing with an etch back process(e.g., reactive ion etch or wet chemical etch) to yield a recessed orpartial plug 32. The distance the partial plug 32 is recessed from thetop surface of the first insulating layer 20 is preferably between about500 Å and 10,000 Å, more preferably between about 1,000 Å and 5,000 Å,and in the illustrated embodiment is between about 1,500 Å and 2,000 Å.Note that the figures are not drawn to scale. Persons skilled in the artwill recognize other methods of forming the via 30 and the partial plug32 that are compatible with the preferred embodiment of the presentinvention. For example, the partial plug 32 can be formed by selectivelydepositing poly into the via 30.

The skilled artisan will readily appreciate that the methods of thepreferred embodiments can also be applied to plug constructions withinthe partial plug. For example, where the contact is to be made to metalor other material less sensitive than the transistor active area,particularly where the via has a less severe aspect ratio than thepreferred embodiments, the entire plug can be formed of platinum orother material in accordance with the preferred embodiments.

Returning to the preferred embodiments, FIG. 2 shows a transition region34, providing ohmic contact between the partial plug 32 and thesubsequent overlying layers. In FIG. 2, this transition region 34comprises a titanium silicide (TiSi_(x)) layer 36 and a titanium nitride(TiN) layer 38. The latter layer serves as an effective diffusionbarrier. One method of fabricating the transition region 34 isself-aligned silicidation, which comprises a blanket deposition oftitanium (Ti), followed by an annealing step in a nitrogen atmosphere.This process forms the TiSi_(x) layer 36 on top of the partial plug 32,with the TiN layer 38 on top of the TiSi_(x) layer 36, leaving thestructure shown in FIG. 2. Alternatively, any excess Ti can beselectively etched away after silicidation and a TiN layer can bedeposited over silicide. Though not shown, it will be understood thatforming the transition region may include deposition of material on thesidewalls of the recessed via 30, which sidewall material can remainthrough to formation of the final structure, depending upon the methodof formation. Persons skilled in the art recognize that there are othermethods of forming the transition region 34 that are compatible with thepreferred embodiment of the present invention.

The recessed or partial plug 32 is then capped with a desired conductivematerial by selectively refilling the recessed portion of the via 30,i.e., without mask steps. The desired material of the preferredembodiments comprises a metal that is difficult to polish usingconventional planarization techniques. Exemplary materials that areparticularly suitable for capping a plug beneath a high k capacitorinclude palladium, platinum, iridium, rhodium, ruthenium, gold andalloys of these materials that form conductive barriers or conductiveoxides that do not degrade in oxygen. (e.g. TiAlN, PtIr, PtRu, PtRh,RuO_(x), IrO_(x), etc.) In the illustrated embodiment, selectiverefilling is accomplished by a sacrificial deposition, polishing forisolation, and a substitution reaction.

FIG. 3 shows a first step in selectively refilling, in accordance withthe illustrated embodiment of the present invention. A sacrificial layer40 is blanket deposited onto the first insulating layer 20 and the via30, resulting in the structure illustrated in FIG. 3. The sacrificiallayer 40 comprises material that is susceptible to a planarizationprocess (e.g., chemical mechanical planarization, dry etch). Thematerial can be readily oxidized and can either be mechanically abraded,or can form a volatile species during an etch. Preferably, thesacrificial layer 40 material is a metal that is less noble than thedesired final refractory metal layer. For example, for a desired caplayer comprising platinum, materials for the sacrificial layer 40include, but are not limited to: copper, tungsten, silver and titanium.In the illustrated embodiment of the present invention, the sacrificiallayer 40 comprises copper deposited by physical vapor deposition (PVD).

By planarizing the sacrificial layer 40 to remove the excess sacrificialmaterial, the sacrificial layer 40 is confined to the via 30, asillustrated in FIG. 4. Such planarization can be accomplished bymechanical abrasion, preferably chemically aided by etchants in a slurryin a chemical-mechanical planarization or polishing (CMP) process.Advantageously, CMP electrically isolates the sacrificial layer 40 inplugs across an array from one another in a self-aligned manner, withoutthe expense of an additional photolithographic mask step. Personsskilled in the art can readily determine an effective slurry chemistryfor aiding the removal of the preferred sacrificial material (copper)while stopping the etch when the first insulating layer 20 is exposed.As will be appreciated by the skilled artisan, a CMP shield or an etchstop layer can also be employed to help define the stopping point forthe planarization.

After the CMP process, a replacement reaction is conducted in which theatoms of the sacrificial layer 40 are replaced by atoms of a differentmaterial, forming a plug cap 50. The illustrated replacement reactioncomprises immersion plating, which utilizes a galvanic displacementreaction between the sacrificial metal and a relatively more noble metalion in solution. Immersion plating baths typically comprise a salt withmetal ions (e.g., ammonium hydroxide- or potassium-based metal salts) ina solvent with chelating agents, buffers and other stabilizers. Byimmersing the sacrificial layer 40 (e.g., copper) in a bath comprisingions of a more noble metal (e.g., platinum), the sacrificial atoms arereplaced by atoms of the more noble metal. Possible choices for thesubstituted noble metal are gold, platinum, silver, iridium, ruthenium,rhodium, osmium, etc. The illustrated embodiment employs ahigh-metal-ion-concentration acid immersion plating bath held atapproximately 65° C. and comprising approximately 20 g/L of platinum asH₂PtCl₆ and 300 g/L of hydrochloric acid. Alternatively, a commerciallyavailable immersion plating bath may be used. For example, Technic, Inc.of Providence, RI has a palladium immersion plating solution called“Pallamerse” and a gold immersion plating solution called “Oromerse N”which are compatible with the present invention. Similarly, othersacrificial metals can be replaced with more noble (higher reductionpotential) metals immersion plating. Persons skilled in the art canreadily determine an effective bath chemistry for immersion plating thesubstitute metal layer in place of the sacrificial layer 40.

The resulting structure, shown schematically in FIG. 5, has a plug cap50 over the first conductive barrier region 34 and confined to the via30. In the illustrated embodiment of the present invention, the plug cap50 comprises platinum, which is conductive and resistant to oxidation.

Utilizing a sacrificial layer 40 and a substitution reaction permits theuse of metals which are difficult to remove by CMP or dry etch, such asplatinum, while still obtaining the processing advantages of usingstandard planarization techniques to isolate the plug cap, rather thanphotolithography. For example, using a standard CMP process to removeexcess platinum is somewhat difficult because the relative softness ofplatinum causes the metal to smear across the CMP-stopping surface, andit does not readily oxydize, such that a mechanical process would beneeded to planarize it. Conventional planarization is therefore notreliable for completely removing platinum and similar materials betweenplugs, risking short circuits between devices. Platinum and other metalsthat are difficult to planarize are particularly useful, however, formaintaining their conductivity, in the face of highly oxidizingenvironments, such as during formation of high-k dielectric layers.Moreover, platinum can be alloyed to form effective diffusion barrieragainst oxygen and other contaminants.

In other arrangements, the plug cap 50 can be selectively formeddirectly on the interlayer region 34. For example, electroless platingcan be employed to selectively form the desired cap within the via 30over the recessed plug 32. Electroless plating is similar to immersionplating in that it is also a controlled autocatalytic reduction of adissolved metal. Unlike inversion plating, electroless plating utilizesa separate reducing agent to prepare the partial plug surface forplating of the cap material.

As noted, the preferred cap can comprise a conductive alloy or compoundthat serves as an effective barrier against diffusion of oxygen or othercontaminants, such as TiAIN, PtIr, PtRu, PtRh, RuO_(x), IrO_(x), etc.Accordingly, the selectively formed cap layer 50 can be optionallyconverted (e.g., oxidized or alloyed) to an appropriate barrier materialprior to completion of the integrated circuit.

In the preferred embodiment illustrated in FIG. 6, the initial,selectively formed plug cap 50 is converted to a barrier plug cap 55comprising a platinum-rhodium alloy (PtRh_(x)). One method of formingthe barrier plug cap 55 is to deposit rhodium over the initial plug cap50, followed by annealing (e.g., heating to 400° C.-700° C. for 2-30minutes). The anneal creates an platinum-rhodium alloy by interdiffusionat the platinum-rhodium interface. Depending upon the amount of rhodiumand length/temperature of the anneal, the platinum can be wholly orpartially alloyed. Persons skilled in the art will recognize that othersuitable barrier materials can be used without departing from the spiritand scope of the present invention. For example, many metal nitrides arealso known to serve effectively as conductive diffusion barriers.

While the subsequent figures illustrate a wholly converted barrier caplayer 55, it will be understood that, in other arrangements, the caplayer can include only a surface barrier layer. Alternatively, theinitial cap layer 50 as formed by the selective process, can be employedwithout further modification.

After the plug cap 50 is formed, a capacitor 60 is fabricated thereupon.In the illustrated embodiment, a second insulating layer 61 is formedover the first insulating layer 20 and the plug cap 50, as schematicallyillustrated in FIG. 7. Desirably, the second insulating layer 61 isthick enough to electrically insulate overlying conductors fromunderlying devices. In the illustrated embodiment, the second insulatinglayer 61 also serves as a structural layer in which a three-dimensional,folding capacitor is to be defined. Preferably, the insulating layer 61comprises an oxide formed from tetraethylorthosilicate (TEOS), which isslightly denser than BPSG. Alternatively, the second insulating layer 61can comprise BPSG, low k material or other suitable insulating material.Preferably, the thickness of the second insulating layer 61 is greaterthan about 500 Å, more preferably greater than about 1,500 Å, and mostpreferably between about 2,000 Å and 3,000 Å.

After the deposition of a second insulating layer 61, a “container”shaped via 62 is etched into the second insulating layer 61 to exposethe plug cap 50, as illustrated in FIG. 8. The illustrated container via62 is substantially cylindrical, with a preferred diameter between about0.15 μm and 0.25 μm. Persons skilled in the art will readily recognize avariety of other capacitor configurations and corresponding methods offabrication for maximizing capacitor surface area.

The container via 62 is then coated with a deposited conductive layer,which forms the bottom electrode 64 of the capacitor. In the embodimentillustrated in FIG. 9, the bottom electrode 64 comprises platinum. Thisbottom electrode 64 has a preferred thickness preferably between about100 Å and 500 Å. Persons skilled in the art will recognize that avariety of materials and processes can be used to form the bottomelectrode 64, including, but not limited to, chemical-vapor deposition(CVD), physical-vapor deposition (PVD) or plating (electroplating,electroless or displacement), followed by patterning (e.g., a dry-etchprocess to remove the excess metal outside the container via 62).

A high k dielectric layer 66 is then deposited into the via, asillustrated in FIG. 10. The high k material preferably has a dielectricconstant of about 10 or greater. Exemplary high k materials includeTa₂O₅, BST, ST, BT, PZT, etc. In the illustrated embodiment, the high kdielectric 66 comprises Ta₂O₅, deposited by MOCVD.

A common feature of such high k materials is the tendency to causeoxidation of surrounding materials, such as underlying plugs. Anysuitable process, including but not limited to CVD, PVD or spin-onprocesses can deposit the high k dielectric layer 66. The preferredembodiments are particularly advantageous in preventing oxidation ofunderlying elements during high temperature, oxygen-rich curing annealsfor filling oxygen vacancies in the high k material 66. The thickness ofthis high-k dielectric layer 66 is preferably between about 15 Å and 250Å.

The top electrode 68 of the capacitor 60 is then formed on top of thehigh k dielectric layer 66, as illustrated in FIG. 11. The top electrodeconsists of a conductive material, typically a metal deposited using oneof a variety of standard processes known to persons skilled in therelevant art. The top electrode 68 can extend over all capacitors in anarray, as shown, or it can be patterned to leave isolated capacitors.

FIG. 12 shows another insulating layer 70 formed over the completedcapacitor 60. As will be appreciated by the skilled artisan, theintegrated circuit can then be completed by a series of metallization orwiring steps, followed by passivation of the chip.

The resultant memory cell structure comprises a MOSFET drain region 16,a capacitor 60, and a via 30 through a first insulating layer 20. Thevia 30 is filled with a conductive plug 80, which electrically connectsthe drain region 16 to the capacitor 60.

The conductive plug 80 comprises a partial polysilicon plug 32, atransition region 34 and a plug cap 55. The partial plug 32 comprises aconductive material is compatible and makes ohmic contact with theunderlying substrate 12. The transition region 34 over the partial plug32 comprises a conductive material which, in the preferred embodimentillustrated in FIG. 12, comprises a titanium silicide layer 36 directlyover the partial plug 32, and a titanium nitride layer 38 over thetitanium silicide layer 36. The transition region 34 extends generallyhorizontally across the plug 80, and intervenes between the underlyingpolysilicon partial plug 32 and overlying cap material 55.

The plug cap 55 overlies the first conductive barrier region 34 andwithin the via 60, preferably filling the remainder of the via 30 overthe partial plug 32 and over the transition region 34. The unitaryinsulating layer 20 thus surrounds both the partial plug 32 and thealigned plug cap 55. Preferably, the cap 55 comprises a material that isconductive and resistant to oxidation. In the illustrated embodiment,the cap 55 comprises a noble metal, preferably platinum. The illustratedbarrier plug cap 55 further comprises a second material that, combinedwith the selectively formed metal, forms an effective barrier to oxygendiffusion. In other arrangements, the barrier material can be formedduring the selective process.

Advantageously, the material of the plug cap 55 has not been planarizedor otherwise etched during the process of fabrication. Rather, thesacrificial layer was etched earlier in the process (namely by CMP), andthat etch step defines the configuration of the final material that issubstituted for the sacrificial layer. Alternatively, selectivedeposition enables formation of the plug cap 55 within the via 30,without the need for etching.

Although described above in connection with particular embodiments ofthe present invention, it should be understood the descriptions of theembodiments are illustrative of the invention and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

We claim:
 1. A method of selectively forming a cap layer for aconductive plug of an integrated circuit, comprising: forming a via inan insulating layer, the via exposing a semiconductor substrate; forminga partial plug partially filling the via and directly contacting thesubstrate; depositing a sacrificial layer into the via, over the partialplug and over the insulating layer, the sacrificial layer in electricalcontact with the partial plug; removing an excess portion of thesacrificial layer from over the insulating layer; and replacing atoms ofthe sacrificial layer within the via with atoms of a cap material. 2.The method described in claim 1, wherein removing comprises planarizing.3. The method described in claim 2, wherein planarizing comprises achemical mechanical polish.
 4. The method described in claim 1, whereinthe sacrificial layer is formed of a material that is readily oxidizedand removed by chemical mechanical planarization.
 5. The methoddescribed in claim 1, wherein the sacrificial layer is selected from thegroup consisting of copper, aluminum, tungsten and titanium.
 6. Themethod described in claim 1, wherein the cap material is resistant tooxidation.
 7. The method described in claim 1, wherein the cap materialcomprises a material selected from the group consisting of gold,platinum, palladium, silver, iridium, ruthenium, rhodium and osmium. 8.The method described in claim 1, wherein replacing comprising conductingan immersion plating process.
 9. The method described in claim 1,further comprising forming a transition region over the partial plug andunder the sacrificial layer.
 10. The method described in claim 9,wherein the transition region comprises a conductive diffusion barrierlayer.
 11. The method described in claim 10, wherein the conductivediffusion barrier layer comprises a metal nitride.
 12. The methoddescribed in claim 9, wherein forming the transition region comprisesblanket depositing a refractory metal over the partial plug andannealing the refractory metal.
 13. The method described in claim 9,wherein the transition region comprises a metal silicide.
 14. The methoddescribed in claim 1, wherein the partial plug comprises conductivelydoped polycrystalline silicon.
 15. The method described in claim 1,wherein forming the partial plug comprises blanket depositing a layerwithin the via and recessing the layer within the via.
 16. The methoddescribed in claim 15, wherein recessing comprises a reactive ion etch.17. The method described in claim 15, wherein recessing comprises a wetchemical etch.
 18. The method described in claim 1, wherein the methodfurther comprises converting the plug cap into a diffusion barrier. 19.The method described in claim 18, wherein converting comprisesdepositing a material selected from the group consisting of iridium,rhodium and ruthenium over the plug cap and alloying the material withthe plug cap.
 20. A method of fabricating a system including anintegrated circuit, comprising: filling a contact via in an integratedcircuit with a silicon plug; recessing the silicon plug within the via;forming a plug cap comprising a noble metal selectively over therecessed silicon plug without etching the noble metal; and forming acapacitor over the plug cap, the capacitor incorporating a material witha dielectric constant greater than about
 10. 21. The method described inclaim 20, wherein the silicon plug contacts a semiconductor substrate.22. The method described in claim 20, wherein the plug cap comprisesplatinum.
 23. The method described in claim 22, further comprisingforming a barrier material from the plug cap.
 24. The method describedin claim 20, wherein forming the plug cap comprises electroless platingthe noble metal over the recessed plug.
 25. A method of fabricating asystem including an integrated circuit, comprising: filling a contactvia in an integrated circuit with a silicon plug; recessing the siliconplug within the via; forming a plug cap comprising a noble metalselectively over the recessed silicon plug without etching the noblemetal; forming a capacitor over the plug cap, the capacitorincorporating a material with a dielectric constant greater than about10; and forming a barrier layer over the recessed silicon plug prior toforming the plug cap.
 26. The method described in claim 25, wherein thebarrier layer comprises a metal nitride.
 27. A method of fabricating asystem including an integrated circuit, comprising: filling a contactvia in an integrated circuit with a silicon plug; recessing the siliconplug within the via; forming a plug cap comprising a noble metalselectively over the recessed silicon plug without etching the noblemetal, wherein the plug cap comprises platinum; forming a barriermaterial from the plug cap, wherein forming the barrier materialcomprises alloying the plug cap with a second metal; and forming acapacitor over the plug cap, the capacitor incorporating a material witha dielectric constant greater than about
 10. 28. A method of fabricatinga system including an integrated circuit, comprising: filling a contactvia in an integrated circuit with a silicon plug; recessing the siliconplug within the via; forming a plug cap comprising a noble metalselectively over the recessed silicon plug without etching the noblemetal, wherein the plug cap comprises platinum; forming a barriermaterial from the plug cap, wherein forming the barrier materialcomprises converting an upper surface of the plug cap to a metal alloy;and forming a capacitor over the plug cap, the capacitor incorporating amaterial with a dielectric constant greater than about
 10. 29. A methodof fabricating a system including an integrated circuit, comprising:filling a contact via in an integrated circuit with a silicon plug;recessing the silicon plug within the via; forming a plug cap comprisinga noble metal selectively over the recessed silicon plug without etchingthe noble metal, wherein forming the plug cap comprises depositing asacrificial metal having a lower potential than the noble metal over therecessed silicon plug, chemical mechanical planarizing the sacrificialmetal, and replacing the sacrificial metal with the noble metal; andforming a capacitor over the plug cap, the capacitor incorporating amaterial with a dielectric constant greater than about
 10. 30. A processfor forming an integrated circuit memory cell, comprising: forming arecessed plug within a via through an insulating layer, the recessedplug electrically connected to a transistor active area; selectivelyforming a cap layer within the via over the recessed plug, the cap layercomprising a non-oxidizing conductive material, wherein selectivelyforming comprises: blanket depositing a sacrificial layer over therecessed plug; polishing the sacrificial layer to leave the sacrificiallayer only within the via; and selectively replacing the polishedsacrificial layer with the cap layer; and forming a capacitor over thecap layer, the capacitor incorporating a dielectric materialcharacterized by a dielectric constant greater than about
 10. 31. Theprocess of claim 30, wherein selectively replacing comprises immersionplating.
 32. A method of forming a plurality of non-oxidizing contactstructures in an integrated circuit, the method comprising: forming aplurality of openings through an insulating layer; blanket depositing asacrificial material over the insulating layer and into the openings;removing the sacrificial layer from over the insulating layer outsidethe openings; and replacing the sacrificial layer inside the openingswith a non-oxidizing material, wherein replacing comprises immersionplating.
 33. The method of claim 32, wherein the non-oxidizing materialcomprises a metal having a higher reduction potential the sacrificiallayer.
 34. The method of claim 33, wherein the metal comprises platinum.35. The method of claim 32, wherein the non-oxidizing material comprisesa oxygen barrier.
 36. The method of claim 32, wherein the non-oxidizingmaterial has characteristics making it difficult to planarize.
 37. Themethod of claim 32, wherein the non-oxidizing material comprises aconductive oxide.
 38. A method of forming a plurality of non-oxidizingcontact structures in an integrated circuit, the method comprising:forming a plurality of openings through an insulating layer; blanketdepositing a sacrificial material over the insulating layer and into theopenings; removing the sacrificial layer from over the insulating layeroutside the openings, wherein removing comprises polishing; andreplacing the sacrificial layer inside the openings with a non-oxidizingmaterial, wherein replacing comprises immersion plating.
 39. A method offorming a plurality of non-oxidizing contact structures in an integratedcircuit, the method comprising: forming a plurality of openings throughan insulating layer; blanket depositing a sacrificial material over theinsulating layer and into the openings; removing the sacrificial layerfrom over the insulating layer outside the openings; and replacing thesacrificial layer inside the openings with a non-oxidizing material,wherein the openings are partially filled with silicon prior to blanketdepositing.
 40. A method of forming a plurality of non-oxidizing contactstructures in an integrated circuit, the method comprising: forming aplurality of openings through an insulating layer; blanket depositing asacrificial material over the insulating layer and into the openings;removing the sacrificial layer from over the insulating layer outsidethe openings; and replacing the sacrificial layer inside the openingswith a non-oxidizing material, wherein the non-oxidizing materialcomprises a conductive oxide.